The death of Benazir Bhutto is an ominous sign not only for the future of Pakistan but also for the course of the U.S.-led war in neighboring Afghanistan, U.S. military officers and other experts on South Asian security affairs said today.

The assassination of the former Pakistani prime minister came at the end of a year in which the Afghan war turned more violent, with a sharp spike in suicide bombings and roadside bombs. Also, the Taliban, ousted from power by a U.S.-led campaign in November 2001, have regrouped and expanded their operations beyond their homeland in Afghanistan's south-central mountains.

"This event is just the latest in a line of disturbing events," said Andrew Exum, who fought in Afghanistan in 2004 as a U.S. Army officer and now studies Islamic militant groups at King's College London.

The prime concern of defense experts is that Pakistani President Pervez Musharraf will have his hands full domestically, and so will be less helpful to the U.S. government, and less able to crack down on the movement of Islamic fighters across Pakistan's ungoverned border with Afghanistan.

A longer-term worry is whether the assassination sets off a chain of events that fractures Pakistan politically, an outcome that would far outstretch the ability of the U.S. military to respond. The major focus then would be securing Pakistan's arsenal of 60 to 100 nuclear warheads and ensuring that some aren't captured by al-Qaeda and its allies.

How the short-term effects play out in Afghanistan depends largely on whether Islamic extremists are found to have carried out the killing of Bhutto, and, if so, how Musharraf reacts. If those groups are found to be behind the killing, then attention will focus on their redoubts in the border areas. Defense Secretary Robert M. Gates expressed concern last week that al-Qaeda has "reestablished itself" in Pakistan's ungoverned area along its border with Afghanistan.

"The big question is, will the Pakistani government be able to do anything there under these circumstances, [ as it is] preoccupied with post-assassination trouble?" said Teresita C. Schaffer, a former State Department official now at the Center for Strategic and International Studies.

The U.S. military has about 26,000 troops in Afghanistan, many of them near the eastern border. Some experts predicted that the killing of Bhutto will encourage Islamic extremist elements in that area.

"They will see this as a victory, and . . . Musharraf looks weak," said John McCreary, who led the Defense Intelligence Agency's 2001 task force on Afghanistan. "Spillover effects will be to embolden the Taliban and tribal miscreants."

If Musharraf escalates operations against al-Qaeda militants in the border areas, that also could complicate the situation in Afghanistan, said J. Alexander Thier, a former United Nations official in Afghanistan. "If done in a significant way, it will stir up the hornets' nest, which may well result in a surge in Afghan violence," he said.

There is also the possibility that the Pakistani government will turn hostile to U.S. interests, which would change the entire nature of the war in Afghanistan.

"If there is a break in U.S.-Pakistan relations due to some ill-considered policy decision on the part of the U.S.," said retired Pakistani Brig. Naeem Salik, now a scholar at Johns Hopkins University, "it wouldn't be possible for anyone in power in Islamabad to continue to allow the transit facilities and the flow of logistic support" for U.S. forces in Afghanistan.